IR-A% denotes the percentage of inherited runners who come around to score against a relief pitcher. It is determined by dividing the inherited runs scored against a pitcher by the total number of runners he has inherited. This statistic essentially asks the question: How often does a relief pitcher allow a runner (or multiple runners) to score when he enters the game with a runner (or multiple runners) on base?
IR-A% isn't a perfect statistic for evaluating relievers, because it's obviously easier to strand runners in certain situations. For instance, it's easier for a relief pitcher to strand a runner who is on first base with one out, than it is for him to strand a runner who is on third base with none out. However, over the course of the season, those circumstances typically even out, and the game's better relief pitchers usually allow the lowest percentage of inherited runners to score.
Any baserunners allowed by a relief pitcher do not factor into his eventual IR-A percentage. Therefore, IR-A percentage has no bearing on a reliever's ERA.
In A Call
"inherited-run rate," "inherited-run-scored rate," "strand rate" (opposite)